1. Field of the Invention
The present invention relates to a magnetic recording medium and more particularly to a magnetic recording medium which enables high-density recording by virtue of a high coercive force and, particularly, exhibits reduced noise, an improved resolution of the reproducing waveforms and an increased S/N ratio. The present invention also relates to a magnetic recording disk device, in brief, a magnetic disk device, for recording and reproducing information, the disk device using the magnetic recording medium of the present invention and being provided with a magnetoresistive head.
2. Description of Related Art
The development of information processing techniques has led to an increasing demand for an increase in the density of magnetic disk devices used in external storage devices for computers. Specifically, in the reproducing head of the magnetic disk devices, the use of a magnetoresistive head utilizing a magnetoresistor, wherein the electric resistance changes in response to the magnetic field intensity, that is, an MR head, instead of the conventional wound-type inductive thin film magnetic head has been proposed in the art. The MR head applies magnetoresistance, that is, the change in electric resistance produced in a magnetic material on application of an external magnetic field, to the reproduction of a signal on a recording medium and has features including a reproduction output margin that is several times larger than that of the conventional inductive thin film magnetic head, a low inductance and a large S/N ratio. Further, the use of an AMR (anisotropic magnetoresistive) head utilizing anisotropic magnetoresistance, a GMR (giant magnetoresistive) head utilizing giant magnetoresistance, and a spin valve GMR head of a practical type, besides the MR head, have also been proposed.
Further, in order to meet the demand for high-density recording, a sufficient improvement in properties, to cope with the above MR head, AMR head, or GMR head (including spin valve head) has been demanded of the magnetic recording medium. In particular, low noise and a high coercive force Hc are required in the magnetic recording medium.
Hitherto, as is well-known in the art, the magnetic recording medium has been generally produced by depositing chrominium on a nonmagnetic substrate such as an aluminum substrate to form an underlayer, followed by depositing a cobalt-based alloy on the resulting chromium underlayer to form a magnetic recording layer.
Further, to obtain a reduced noise level, many changes, such as the addition of an additional element to the alloy of the magnetic recording layer, thereby breaking the magnetic interaction between the magnetic particles or reduction in the particle size of the magnetic particles, have been made to the magnetic recording medium. For example, Japanese Unexamined Patent Publication (Kokai) No. 63-148411 discloses a low noise-and high density recording-type magnetic recording medium which is suitable for use in high density recording devices. The magnetic recording medium disclosed in this publication is characterized in that the Co/Ni-based alloy or Co/Cr-based alloy constituting the magnetic recording layer contains a third element added thereto, that is, any one of Ta, Mo and W or an alloy thereof. Japanese Unexamined Patent Publication (Kokai) No. 7-50008 discloses a magnetic recording medium which can simultaneously satisfy both the requirements of a high coercive force and low noise. Specifically, the magnetic recording medium disclosed in this publication comprises a nonmagnetic substrate layer and a magnetic layer deposited through a nonmagnetic metal underlayer (thickness=10-300 nm) of chromium or a chromium alloy on the nonmagnetic substrate layer, the magnetic layer being formed of an alloy containing Co, Cr, Pt, and at least one member selected from the group consisting of Nb, Hf, W, Ti, and Ta. According to this invention, not only a high coercive force of 1,610 to 1,750 Oe (Examples 1 to 7) but also low noise can be achieved. Similarly, Japanese Unexamined Patent Publication (Kokai) No. 7-50009 discloses a magnetic recording medium wherein a thin film medium of an alloy of 95 to 60 at % of Cr and 5 to 40 at % of at least one member selected from Mo and W is used as an underlayer for a magnetic layer, having a single layer structure, of a CoCrPt alloy. This magnetic recording medium also can realize, simultaneously, a high coercive force and low noise. More specifically, the use of a Cr layer containing 28 at % of Mo as the underlayer results in an about 10% reduction in noise as compared with the underlayer consisting of Cr alone. In the techniques disclosed in these publications, however, tBr (a product of the thickness t and the residual magnetic flux density Br of the magnetic recording layer) is not less than 270 G..mu.m, rendering these techniques unsatisfactory for the higher density recording expected in the future.
In addition, to attain a highly increased density in the magnetic recording devices, it is necessary to provide a magnetic recording medium having a high resolution of signal in which a reproducing output is not easily reduced at the high recording density, and thus the development of such medium has been zealously sought in the field of magnetic recording. For example, in the reference material 91-8-3(2) published on 1991 by Jikikougaku Kenkyukai (magnetic engineering research association), a magnetic recording medium capable of both high resolution and low noise has been reported. According to this report, a high output and low noise magnetic recording medium can be produced by first depositing a low noise material, Co.sub.74.2 Cr.sub.14.5 Pt.sub.8.4 Si.sub.2.9, on a Cr underlayer to form a lower magnetic layer, followed by directly depositing a high output material, Co.sub.74.2 Cr.sub.4.7 Pt.sub.21.1, on the lower magnetic layer to form an upper magnetic layer. However, for this duplicated magnetic layer of Co.sub.74.2 Cr.sub.4.7 Pt.sub.21.1 /Co.sub.74.2 Cr.sub.14.5 Pt.sub.8.4 Si.sub.2.9, it is difficult to obtain a remarkably improved S/N ratio due to the use of CoCrPt alloy with a low Cr content, as an upper magnetic layer.
In this connection, the inventors have measured the read/write performances for the above-discussed magnetic recording media and other conventional magnetic recording media, and found that sufficiently high read/write performances cannot be attained when the recording density is the level required at present, that is, generally 1 Gb/in.sup.2 or more. In other words, for these magnetic recording media, it becomes possible to obtain low noise but it is difficult to avoid drawbacks which are caused simultaneously, such as a reduction of reproducing output. It is therefore desirable to ensure a remarkable reduction of noise in the media, while maintaining a high reproducing output.
For reference, Japanese Unexamined Patent Publication (Kokai) No. 60-228637 discloses a CoCr alloy which is useful as a target material in the production of a magnetic recording medium using a sputtering method. Using this target material, cracks can be effectively prevented in the resulting magnetic layer and, also, the magnetic layer can be processed with a high efficiency. The CoCr alloy is characterized by containing 9.0 to 22.5% by weight of Cr, 0.001 to 5% by weight of one element or two or more elements selected from the group consisting of Sc, V, Nb, Ta, W, Mn, Tc, Re, Fe, Os, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, B. Al, Ca, In, Tl, C, Si, Ge, Sn, Pb, P, As, Sb, Bi, S, Se and Te, and a balance of Co. However, this publication teaches only that the described sputtering target can inhibit the generation of cracks in the magnetic layer as a function of the addition of a minor amount of additional element(s) to the CoCr alloy, and is silent concerning provision of a magnetic recording medium having a reduced noise, improved resolution of the reproducing waveforms and increased S/N ratio. It should be also noted that the appended working examples of this publication include only the preparation examples of five alloy targets, i.e., CoCrCu alloy, CoCrMnV alloy, CoCrGeSn alloy, CoCrCaSi alloy and CoCrCuC alloy.